Field of the Invention
The present invention relates to an interconnection substrate, and in particular, to an interconnection substrate that includes a transmission line including through-hole interconnections.
Description of the Related Art
In recent years, there has been demand for high-speed data communication between storages or servers at a data transfer rate of tens of Gbps. In such high-speed transmission technology for digital signals, a differential transmission method is often used.
The differential transmission method is a transmission method for transmitting signals of opposite polarities to two parallel lines. This method has a feature in that resistance to common mode noise is high because a signal is recognized based on the potential difference. The interconnection substrate including differential transmission lines is disclosed in Japanese Patent No. 2736107, for example.
The differential transmission method is advantageous in terms of low power consumption, high resistance to external noise, and less susceptibility to the influence of ground potential. Such a data transmission system is configured to include a signal transmission IC (transmitter), a channel (signal transmission path), and a signal receiving IC (receiver).
The wiring design rule of the terminal spacing of an IC is different from that of the terminal spacing of a channel, such as a printed circuit board. Therefore, an interposer substrate (interposer substrate) is often interposed between the IC and the channel.
FIGS. 11A and 11B are an example of a conventional interposer substrate 100 including through-hole interconnections through which differential signals flow. A signal emitted from an IC 120 passes through a surface wiring, which is formed on the main surface of the interposer substrate 100, and then passes through a through-hole interconnection 110 inside the interposer substrate and is guided to a printed circuit board 130 through a mounting portion. As described above, the surface wiring and the through-hole interconnection through which differential signals flow are two parallel lines.
FIG. 12 shows a conventional transparent perspective view of an interposer substrate including through-hole interconnections through which differential signals flow. If true differential transmission mode signals flow, it can be considered that two through-hole interconnections are coupled by an electric field. In addition, in order to realize excellent transmission characteristics, the differential transmission line is designed to have specific differential impedance (for example, 100 Ω). One of the factors that determine the differential impedance is a separation distance between two parallel through-hole interconnections. Generally, forming through-hole interconnections penetrating through the substrate is technically more difficult than forming surface wirings formed on the substrate surface. Therefore, the through-hole interconnections are thicker than the surface wirings. In addition, with regard to the differential transmission line configured to include the through-hole interconnections, in order to realize target differential impedance, it is necessary to ensure a distance between two parallel through-hole interconnections to some extent. In other words, under restrictions to realize excellent transmission characteristics, it is not possible to narrow the distance between two through-hole interconnections. Therefore, it is necessary to ensure a region occupied by two through-hole interconnections in the substrate to some extent or more. For this reason, there has been a problem in that it is difficult to miniaturize the substrate and to form high-density through-hole interconnections.
The aforementioned conventional problem will be described in more detail using the transmission line theory. A TEM mode is known as a transmission mode of the transmission line, such as a microstrip line or a coaxial line.
Here, the transmission of the TEM wave will be discussed.
Generally, the TEM mode is a high-frequency signal transmission mode between two conductors. For example, one of the two conductors is a signal line and the other conductor is a GND.
In the case of a coaxial cable, the former is the core and the latter is an outer conductor.
In the signal line, inductances of L per unit length [H/m] are distributed in series, and the capacitance of C [F/m] per unit length is formed between two conductors. In addition, series resistance R [Ω/m] due to the resistance of the conductor and parallel conductance G [S/m] that defines the amount of signal leakage between two conductors are distributed in the signal line.
In the process of solving the telegraph equation of the transmission system that is being discussed currently, the characteristic impedance Zx is defined as the ratio of voltage V and current I at a certain point, and this can be expressed by the relational expression shown below.
                              Z          x                =                                            R              +                              j                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                L                                                    G              +                              j                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                C                                                                        [                  Math          .                                          ⁢          1                ]            
Here, j is an imaginary unit, and ω is an angular frequency (rad/s) of the AC current.
When the frequency is high and/or the resistance R of the conductor and the dielectric loss G are simply small in this expression, R<<ωL and G<<ωC are satisfied. Accordingly, the expression is simplified as Math. 2.
                              Z          x                =                              L            C                                              [                  Math          .                                          ⁢          2                ]            
That is, the characteristic impedance Zx can be defined by the inductance L of the conductor line and the capacitance C between two conductors.
The differential through-hole interconnections will be discussed from the point of view of differential transmission line design. Because of the differential operation of through-hole interconnections, it is easily expected that the through-hole interconnections are in a form of two parallel lines. In addition, since the through-hole interconnections are considered to be the signal transmission mode between two conductors described above, the above discussion can be directly applied.
In a typical printed wiring board, through holes of through-hole interconnections are manufactured by using a processing method, such as drilling or laser machining, and the diameter depends on the processing method. Generally, the diameter of the through hole of each through-hole interconnection is expected to be about 300 μm. The length of each through-hole interconnection is determined by the substrate thickness. In addition, a substrate has a dielectric constant specific to the material.
In differential design, the differential impedance Zdiff=100 Ω is generally applied in many cases. The differential impedance is the characteristic impedance that has been discussed so far. Accordingly, the impedance is determined by the inductance per unit length and the capacitance per unit length.
According to conventional techniques, it is believed that the inductance of the through-hole interconnection is determined by the diameter of the through hole and the surface area (facing area) of a portion where the through holes face each other and the dielectric constant of the material, which define the capacitance, are also uniquely determined by the substrate technique that is used. Therefore, in order to obtain the target Zdiff=100 Ω in the differential design, it is thought that there is no other way but to adjust the distance between the through holes.
In general, however, each through hole has a large surface area due to the processing size. For this reason, in order to obtain a relatively high impedance of 100 Ω, the through holes should be separated from each other to some extent. For example, the separation distance between the through holes is about 350 to 400 μm, which is larger than the diameter of each through hole.
In order to obtain the target impedance, this separation distance is uniquely determined by the result of the discussion mentioned so far. Therefore, since the distance between differential through-hole interconnections is determined by the design value of differential impedance, there is a problem in that the distance between the differential through-hole interconnections cannot be changed.
In particular, this means that, when a plurality of differential terminals are disposed, the footprint cannot be reduced, which is contrary to the high-density mounting of recent years.
Here, FIG. 13 is a cross-sectional view showing the conventional differential through-hole interconnections.
As shown in FIG. 13, in the conventional differential through-hole interconnections, when the redundancy of the diameter D and height H of through holes 111A and 111B and dielectric material properties (dielectric constant ∈) is small, a distance X between differential through-hole interconnections can have only one value in order to satisfy the specific differential impedance that is defined by differential transmission line design.
In addition, the apparent inductance per unit length (here, defined as the thickness of a substrate) of the through holes 111A and 111B can be expressed by the diameter D [mm] and the height H [mm] of the through hole as in Math. 3.
                    L        =                  0.2          ⁢                                          ⁢                                    H              ⁡                              (                                                      2.303                    ⁢                                                                                  ⁢                    log                    ⁢                                                                  4                        ⁢                                                                                                  ⁢                        H                                            D                                                        -                  0.75                                )                                      ⁡                          [                              n                ⁢                                                                  ⁢                H                            ]                                                          [                  Math          .                                          ⁢          3                ]            
In addition, the capacitance per unit length C between the through holes 111A and 111B is expressed by Math. 4 when the facing area between the through holes 111A and 111B is defined as S and the dielectric constant of the dielectric filled in the through holes 111A and 111B is defined as 107 .
                    C        =                  ɛ          ⁢                      S            X                                              [                  Math          .                                          ⁢          4                ]            
Here, there is only one C0 satisfying the differential impedance Zdiff=√L0/C0=100 Ω. That is, the distance X between through holes cannot be changed.
That is, in the conventional differential through-hole interconnections, the distance X between the differential through-hole interconnections 110A and 110B is large. For this reason, there has been a problem in that the footprint is increased.
The present invention has been made in view of such circumstances of the related art, and it is an object of the present invention to provide an interconnection substrate in which the distance between a pair of through-hole interconnections can be reduced and accordingly transmission lines disposed at a high density can be realized while suppressing the degradation of transmission characteristics.